The present invention claims priority from Japanese Patent Application No. 9-047334 filed Feb. 14, 1997, the content of this being incorporated herein by reference.
1. Field of the Invention
The present invention relates to a color conversion processing in a color management system for realizing a color reproduction of a color image between different color devices such as color scanner, color CRT monitor and color printer, etc., and, particularly, a color conversion technique to be used in converting an arbitrary color in an original color space into a color in an objective color space while making a visual color in one colormetric having a reference white coincident with a visual color in another colormetric having a reference white different from the reference white of the one colormetric.
2. Description of Related Art
In order to realize a color reproduction of a color image between different color image devices such as color scanner, CRT monitor and printer, etc., it is necessary to know color characteristics of the respective color image devices and to use a color conversion utilizing the same color characteristics. In a color reproduction method which is used currently, a color conversion processing is performed by measuring color characteristics of respective color image devices by utilizing a spectrophotometer, etc., and making the measured data coincident in a XYZ colormetric stipulated by CIE (Comission Internationale de 1Eclairage) or a uniform color space L* a* b*.
A case where a color image displayed on a CRT monitor is output by a printer will be described.
In a CRT monitor, the color reproduction using the additive color mixture of three primary color lights of red (R), green (G) and blue (B) is performed. Since the linearity is established in the additive color mixture, the color characteristics of the CRT, that is, the color gamut thereof, is obtained if tristimulus values (XYZ values) of the RGB fluorescent materials and tristimulus values of reference white set in the CRT monitor are obtainable by measurements.
On the other hand, in a printer, it is impossible to obtain the color gamut by a simple method since a complicated color reproduction such as subtractive mixture of colors using a plurality of color inks or parallel mixture of colors, etc., is performed. Currently, the color gamut of the printer is obtained by producing a plurality of color patches controlled by color inks and obtaining XYZ (or L*a*b*) values by measuring colors of the color patches one by one by means of a spectrophotometer. Illumination used in this measurement is set to D50.
Here, the reference white set in each color image device should be considered. When color temperature of reference white set in a CRT monitor is 5000K which is substantially the same as that of a illumination light source used in the printer, XYZ value representing the color gamuts of the CRT and the printer can be utilized as the same color space and, therefore, it is possible to compare the color gamut in XYZ colormetric of the CRT monitor with the color gamut of the XYZ colormetric of the printer directly. That is, for an arbitrary color (Xd, Yd, Zd) on the CRT monitor which exists in the color gamut of the printer, it is possible to make an output color of the CRT monitor coincident with an output color of the printer by merely searching a set of amounts of color inks reproducing the color (Xd, Yd, Zd) without processing the tristimulus value.
In general, however, color temperature of the reference white set in the CRT monitor is very high, that is, as high as, for example, 9300K. When human who is familiar with the white color of D50 looks white set to 9300K, he feels it bluish. Therefore, in order to realize color reproduction between different color image devices having different color temperatures of reference white, the difference in white color must be absorbed.
The conventional color conversion between different reference whites will be described by taking a case of conversion from an arbitrary color (XA, YA, ZA) in a color space of a device A having XYZ value (XWA, YWA, ZWA) of reference white into a color (XB, YB, ZB) in a color space of a device B having XYZ values (XWB, YWB, ZWB) of the reference white as an example.
The color (XB, YB, ZB) of the device B corresponding to the color (XA, YA, ZA) of the device A is calculated by utilizing a ratio of the XYZ values of the reference whites in the devices A and B according to the following equations:
XB=XAxc3x97(XWB÷XWA)xe2x80x83xe2x80x83(1)
YB=YAxc3x97(YWB÷YWA)xe2x80x83xe2x80x83(2)
ZB=ZAxc3x97(ZWB÷ZWA)xe2x80x83xe2x80x83(3)
The color conversion method using the ratios of XYZ values of reference whites is simple and can realize a high speed color conversion.
However, in the case where the color temperatures of reference whites of color image devices such as the CRT monitor and the printer are substantially different, human chromatic adaptation in different viewing environments is complicated. Therefore, colors reproduced according to the conventional color converting technique in which the color conversion is performed by using the ratio calculation of the XYZ values of the reference whites of the color image devices appear substantially differently.
Other than the above mentioned technique, the color appearance model of Hunt has been known (cf. R. W. Hunt, xe2x80x9cRevised Colour-Appearance Model for Related and Unrelated Coloursxe2x80x9d, Color Research and Application, Vol. 16, No. 3, June 1991).
The color appearance model of Hunt controls the human chromatic adaptation by severely setting a color viewing condition and, therefore, it has been considered that it can realize a good color reproduction between color image devices having substantially different color temperatures of reference white. However, in the engineering view point, when this model is utilized, it is difficult to systemize the model due to problems that the monitoring condition for controlling human chromatic adaptation is too severe, that the calculation is so complicated and that the number of parameters necessary for color conversion is too many, etc.
An object of the present invention is to provide a color conversion method which makes a human color vision for a color reproduced by one color image device the same as that for the color reproduced by another color image device having a reference white which is different from that of the one color image device without necessity of constraint of special viewing condition which substantially influences the human chromatic adaptation while requiring minimum amount of calculation.
Another object of the present invention is to provide a color converter which makes a human color vision for a color reproduced by one color image device the same as that for the color reproduced by another color image device having a reference white which is different from that of the one color image device without necessity of constraint of special monitoring condition which substantially influences the human chromatic adaptation while requiring minimum amount of calculation.
In order to achieve the above objects, a color conversion method according to a first aspect of the present invention, which converts an arbitrary color in an original color space into a color in an aimed color space while maintaining these color appearances the same, comprises the steps of restoring a spectral power distribution of an original reference white of the original color space from a color temperature of the original reference white, restoring a spectral power distribution of an aimed reference white of the aimed color space from a color temperature of the aimed reference white, restoring a surface reflectance of the arbitrary color in the original color space by utilizing tristimulus values of the arbitrary color in the original color space, the spectral power distribution of the original reference white and a human color matching function, and obtaining tristimulus values of the color in the aimed color space from the restored surface reflectance, the restored spectral power distribution of the aimed reference white and the human color matching function.
Further, according to a second aspect of the present invention, in order to make possible to perform a color conversion while making human color vision of an arbitrary color in an original color space the same as that of a color in an aimed color space when color temperatures of reference whites of the original color space and the aimed color space are unknown and chromaticity of the reference whites of the original color space and the aimed color space are known, a color conversion method for converting the arbitrary color in the original color space into the color in the aimed color space while maintaining these color appearances the same, comprises the steps of obtaining a correlated color temperature of the original reference white of the original color space from chromaticity of the original color space, restoring a spectral power distribution of the original reference white from the thus obtained correlated color temperature, obtaining a correlated color temperature of the aimed reference white of the aimed color space from the chromaticity of the aimed reference white, restoring a spectral power distribution of the aimed reference white from the thus obtained correlated color temperature, restoring a surface reflectance of the arbitrary color in the original color space by utilizing tristimulus values of the arbitrary color, the spectral power distribution of the original reference white and a human color matching function, and obtaining tristimulus values of the color in the aimed color space from the restored surface reflectance, the restored spectral power distribution of the aimed reference white and the human color matching function.
A color converter according to a third aspect of the present invention which displays an arbitrary RGB data displayed on an original color image display performing a color reproduction by means of RGB lights on an aimed color image display performing a color reproduction by means of RGB lights while maintaining these color appearances the same, comprises spectral power distribution restoring means for restoring a spectral power distribution of a reference white of the original color image display from a color temperature of the reference white of the original color image display and restoring a spectral power distribution of a reference white of the aimed color image display from a color temperature of a reference white of the aimed color image display, spectral surface reflectance restoring means for restoring a surface reflectance in a scene of the arbitrary RGB value displayed on the original color image display on the basis of the restored spectral power distribution of the reference white of the original color image display, the RGB value and the human color matching function and RGB value calculation means for calculating RGB value in the aimed color image display from the surface reflectance restored by the spectral surface reflectance restoring means, the spectral power distribution of the reference white of the aimed color image display restored by the spectral power distribution restoring means and the human color matching function.
In this color converter, the spectral power distribution restoring means restores the spectral power distribution of the reference white of the original color image display from the color temperature of the reference white of the original color image display and restores the spectral power distribution of the reference white of the aimed color image display from the color temperature of the reference white of the aimed color image display, the spectral surface reflectance restoring means restores the surface reflectance in the scene of the RGB value on the basis of the spectral power distribution of the reference white of the original color image display restored by the spectral power distribution restoring means, the arbitrary RGB values displayed on the original color image display and the human color matching function and the RGB value calculation means calculates the RGB values in the aimed color image display from the surface reflectance restored by the spectral surface reflectance restoring means, the spectral power distribution of the reference white of the aimed color image display restored by the spectral power distribution restoring means and the human color matching function.
A color converter according to a fourth aspect of the present invention which displays an arbitrary RGB data displayed on an original color image display performing a color reproduction by means of RGB lights on an aimed color image display performing a color reproduction by means of RGB lights while maintaining these color appearances the same, comprises correlated color temperature calculation means for calculating a correlated color temperature of the reference white of the original color image display from chromaticity of the reference white of the original color image display and a correlated color temperature of the reference white of the aimed color image display from chromaticity of the reference white of the aimed color image display, spectral power distribution restoring means for restoring a spectral power distribution of the reference white of the original color image display from the correlated color temperature of the reference white of the original color image display calculated by the correlated color temperature calculation means and a spectral power distribution of the reference white of the aimed color image display from the correlated color temperature of the reference white of the aimed color image display calculated by the correlated color temperature calculation means, spectral surface reflectance restoring means for restoring a surface reflectance in a scene of the arbitrary RGB value displayed on the original color image display on the basis of the spectral power distribution of the reference white of the original color image display restored by the spectral power distribution restoring means, the RGB value displayed on the original color image display and the human color matching function and RGB value calculation means for calculating RGB value in the aimed color image display from the surface reflectance restored by the spectral surface reflectance restoring means, the spectral power distribution of the reference white of the aimed color image display restored by the spectral power distribution restoring means and the human color matching function.
In this color converter, the correlated color temperature calculation means calculates the correlated color temperature of the reference white of the original color image display from the chromaticity of the reference white of the original color image display and the correlated color temperature of the reference white of the aimed color image display from the chromaticity of the reference white of the aimed color image display, the spectral power distribution restoring means restores the spectral power distribution of the reference white of the original color image display from the correlated color temperature of the reference white of the original color image display calculated by the correlated color temperature calculation means and restores the spectral power distribution of the reference white of the aimed color image display from the correlated color temperature of the reference white of the aimed color image display calculated by the correlated color temperature calculation means.